timing/dwell gurus...help!

[bigchillcar]

it's a stock 150 hp '74 911 with the 2.7 liter...

i recently set my timing by my ear (didn't have a light handy) when i changed my points and set dwell.

today, i got hold of a timing light, set 5 degrees off of top dead center, per specs, unplugged vacuum and set dynamic at 35 degrees at 6k, then reattached vacuum...and drove...

wtf...where did my power go? it's certainly driveable and seems to idle ok, but the peak power is gone...usually get a burst in the 3800-4000 range...not now...

any thoughts?? yes, i've done this job on this car before without problems...hmmm...
ryan

[Ron Broman]

You may want to check your point gap. Unless the dwell is on the money you will never get your timing right

[bigchillcar]

true enough, but i set the points gap at .305, right where it ought to be and the measured dwell turned out to be 36 degrees...specs say 38 plus or minus 3...so i'm right about there.

i agree, it is tough to get the gap set...tiny adjustments can easily make it too small or large a gap.
ryan

[Early_S_Man]

0.014" is the gap you want ...

[bigchillcar]

warren,
i didn't include units...i have the gap at .305 mm or .012 inches like you said.
ryan

[Superman]

If someone who does not know what they are doing sets timing "by ear," timing will be way too advanced, the car will run like a scalded cat right up until the first piston gets a hole in it from the detonation.

[bigchillcar]

superman,
yeah, i used to set timing on an old jeep i had by ear, but it was easy to find the sweet spot...the 911 is different for sure...seems ok over a much wider range. and to address that, yes...my ear setting was much too advanced...it was running like a scalded ape, but glad i've adjusted it back. just think i have it too retarded now even though it's at the mark.

i just recalled too...when i checked dynamic advance at 6k, i unplugged the vacuum line, but failed to plug it...
ryan

[bigchillcar]

thinking now that i may need to go advance the timing a few degrees and see what happens. i don't guess mixture could be playing a role now? maybe too lean? fishing...
ryan

[Early_S_Man]

Ryan,

Might be worthwhile to check the advance curve at various rpm levels from the spec book ...

The values in the following thread apply to your distributor, which has identical specs to the 2.4 911T unit.

distributor lube...

The 'basic' setting at idle is adjusted to the final value at 6000 rpm ... if your idle timing changes very much after setting at 6000 rpm, then there's suspicion about the weights, springs, and lubrication at the pivot posts for the weights, or the phenolic sliding surface for the weights ... i.e., there is probably some sticking going on, so a thorough cleaning and inpection is needed!

[bigchillcar]

indeed, warren...it seems we have the same little book, the technical specifications manual...i was lucky enough to pick mine up on e-bay for my '74 model. your pages look virtually identical. i'll be trying this approach tomorrow...forgot to plug the vacuum today when i disconnected from the advance mechanism during my total advance check at 6k as well...so, i need to recheck everything anyway.

think i should stick with timing at 5 degrees atdc? i've heard stories of some of the guys pushing the static advance upwards to even 12 degrees atdc. if so, increase the octane at the pump? boy she's sluggish right now, none of that great torque feel i always have around 4k in it's present condition!
ryan

[bigchillcar]

whaddaya know...i just had posts #911 and 912...



ryan

[bigchillcar]

ok, car seems to be running fine now after making adjustments.

i checked timing again this am at 900 rpm's (vacuum connected)...still reading 5 degrees atdc, per specs (at the notch just left of the 'Z1')...seemed odd to me that the adjusting nut was all the way to the right on the distributor adjustment though...

so, i did what 'sounds' like advancing, by turning distributor more to the right (not goes left), until i have approximated 5 degrees the other side of Z1...drives normally now, but isn't this 5 degrees btdc now? why did the engine sound like it was advancing by going to 'btdc'? am i just confused here? shouldn't retarding it make it go to 'btdc'? thought i understood this properly...why am i confused??
ryan

[bigchillcar]

bueler? anyone..anyone?

[Superman]

Ryan, I dunno. I think your distributor shaft turns in a different direction from mine. My rotor turms counterclockwise I believe, so turning my distributor body clockwise advances timing. Yours may be opposite. I was not particularly worried about your engine, but my remark was for general consumption. Carburetors are settable "by ear" to some degree, but timing that way is frought with peril.

I think JW likes to time these things (911s) at full advance. He tries for 35 degrees BTDC at full advance (like, 5000 rpm or 6000) and if it knocks, he backs it off. Knocking happens under load, so a drive is necessary. A perfectly safe way to time pretty much any engine is to (under load) advance timing until it just starts to knock, then back it off a couple of degrees. I'll probably get flamed for that remark, but engines of all kinds have been timed that way for decades with good results.

[johnsjmc]

BTDC is advancing. Before top dead center means the spark occurs in advance of the piston reaching the top. The idea is to have flame propagation reach a max just after the piston changes direction to achieve the max pressure(BMEP) downwards on the crank.As it takes TIME for the flame to burn it must be lit earlier when the engine is going faster. The problem with setting by ear is that inaudable detonation may wreck havoc without hearing knock. Some dyno research points to the most efficient timing values to be short of audable detonation as well.

[Superman]

John is right. Also, as you advance timing, engine speed will advance well past the point where it should be set. So, if someone times a car at idle by advancing the timing until the engine speed maxes out, timing will be WAY advanced, and at full advance, detonation will occur.

[Early_S_Man]

Ryan,

Your rotor turns clockwise, so turning dist. body to the right is retarding the setting.

My suggestion is to remove and plug the vac. line to the distributor.

It will be a lot easier if you have an advance-type timing light, but the fixed-type will work, too.

Set the timing to 36° - 38° BTDC at 6000 rpm, then check the advance curve per the spec book at 1000, 1500, 2000, 4000, & 6000 rpm. Your engine should run 'better' without the vacuum retard, on an idle setting of 5° - 10° BTDC ... which is where you should be [without vac. hooked up] if the advance mechanism is working properly!

[bigchillcar]

john,
duh...brain fart i guess...i forgot that 'btdc' means advancing...for some reason, i was associating 'after' with advance. smart...

warren and superman,
yes, turning mine clockwise does retard timing. i have it set at 5 degrees btdc with vacuum connected and it doesn't ping or miss under load...dwell is right on target at 38 degrees.

total advance check is difficult, because i work alone in the boonies (think arkansas), but quick forays to 6k trying to keep an eye on the tach and the timing light look about right at the 35 degree mark.
ryan

[TSwede]

Hi all - car running just fine then Prtronix fitted yesterday.. Put dizzy back pointing to TDC then car not running - moved dizzy until running within it's range of adjustment then started car and ready to set timing with the light...

Timing set correctlr (5 deg adv. fro '75 2..7s.) and car running crap!!!!

Timing estimated and ran beautiful though ticked over at 1100 revs - so I think too advanced..

I've seen this problem before in posts but no consistent cure - anyone else experienced it with pertronix on..

If I have the dizzy a tooth out but it is still adjustable within range does that mean the pulley marks will be thrown out?? and if I run a bit advanced (5 degrees or so) will it blow my engine - didn't hear it pinking and it wasn't running hot)..

Please advise - I'm struggling here...